1. Field of the Invention
The present invention relates generally to Liquid-Crystal Display (LCD) device. More particularly, the invention relates to a LCD device that makes it possible to achieve wide viewing angle and fast response, and a method of fabricating the device.
2. Description of the Related Art
Conventionally, LCD devices have been extensively used for electronic equipment designed for so-called Office Automation (OA) and portable communication and/or information-management terminals. This is because LCD devices have the advantage of compact, thin, and low power consumption.
With LCD devices, the alignment or orientation of liquid crystal molecules is changed by a voltage applied across the liquid crystal layer to thereby control the penetration of light utilizing a variety of the optical properties of the liquid crystal cell, such as the birefringence effect and rotary polarization. LCD devices are classified into various display types according to the utilization way or manner of the optical properties of the liquid crystal cell. In any type of LCD devices, existing important problems to be solved are to improve the viewing angle characteristics and to enhance the response characteristics that make it possible to cope with displaying moving pictures.
For example, with the LCD device of the Twisted Nematic (TN) type, which has been widely used so far, the orientation vector of liquid crystal molecules is changed from the “parallel” state or “white” displaying state where the molecules are parallel to the substrates toward the “black” displaying state according to the magnitude of a voltage applied. However, due to the peculiar actions to the liquid crystal molecules under application of the voltage, there is a problem that the obtainable viewing angle is narrow. This problem is observed remarkably in the rising direction of the molecules when displaying medium tones.
To solve this problem of the narrow viewing angle, various measures have been developed and proposed. For example, the Japanese Non-Examined Patent Publication Nos. 4-261522 published in 1992, 6-43461 published in 1994, and 10-333180 published in 1998 disclose the measures called the “dual domain” or “multiple domain” method. With these methods, a liquid crystal cell containing homeotropically aligned liquid crystal molecules is formed. This cell is sandwiched between two polarizer plates whose polarization axes are fixed to be perpendicular to each other. An oblique electric field is generated in each pixel by using a common electrode with opened portions. Thus, each pixel is formed by two or more liquid crystal domains, thereby improving the viewing angle characteristics. In particular, with the technique disclosed by the Publication No. 4-261522, the alignment of the liquid crystal molecules under application of a voltage is controlled to realize high contrast.
Other examples of these measures are disclosed in the Japanese Non-Examined Patent Publication Nos. 6-43461 published in 1994 and 5-113561 published in 1993. These examples utilize optical components such as an optical compensator plate and a quarter wavelength plate to compensate the birefringence effect of liquid crystal, thereby improving the viewing angle characteristics and/or expanding the viewing angle. With the measure disclosed by the Publication No. 5-113561, quarter wavelength plates are used in addition to an optical compensator plate with a negative axis. These quarter wavelength plates are combined together in such a way that the first one of these plates has a positive optical anisotropy and the second one thereof has a negative one to cancel their own birefringence effects, thereby expanding the viewing angle.
Moreover, the Japanese Non-Examined Patent Publication No. 4-502524 published in 1993 discloses a LCD device of the In-Plane Switching (PS) type. With this device, a voltage is applied across a pair of opposing electrodes provided on the same substrate to generate electric fields parallel to the substrates, thereby rotating the liquid crystal molecules while keeping the orientation of the molecules parallel to the substrates. Since there is no possibility that the molecules are oriented perpendicular to the substrates even when a voltage is applied, the birefringence change according to the change of the viewing angle is restrained within a narrow range. As a result, the viewing angle is increased.
The above-described prior-art techniques are to improve the viewing angle characteristics of LCD devices. Unlike this, the Japanese Non-Examined Patent Publication Nos. 10-142577 and 10-197844 both published in 1998 disclose LCD devices capable of switching between wide and narrow viewing angles.
With the device disclosed by the Publication No. 10-142577, two Thin-Film Transistors (TFTs) are provided for each pixel in the TN mode and at the same time, each pixel is divided into two regions where the liquid crystal molecules are rotated or inclined at different angles, thereby widening the viewing angle like the so-called “capacitance division” method. When the device is operated at the narrow viewing angle, one of the two TFTs is turned on to operate in the ordinary TN mode. When the device is operated at the wide viewing angle, both of the two TFTs are turned on to operate in the modified TN mode.
With the device disclosed by the Publication No. 10-197844, a Guest-Host (GH) element containing a dichroic pigment is stacked on the ordinary TN element. A voltage is applied across the GH element to control the viewing angle characteristics.
As explained above, the viewing angle characteristics of LCD devices are improved by using the above-described techniques. However, there are still many problems on improvement of fast response of LCD devices. In general, one of the known measures to improve the response speed of LCD devices is to increase the magnitude of a voltage applied across the liquid crystal molecules. In this case, however, if the cell gap is reduced, the capacitance of the liquid crystal increases. Thus, a problem is likely to occur. In particular, if the LCD panel is large-sized, a problem of signal transmission delay along the wiring lines and/or insufficient writing voltage to the TFTs will occur.
Another one of the known measures to improve the response speed of LCD devices is to increase the changeable range of transmittance of the liquid crystal molecule with respect to the inclination of the molecules by using a liquid crystal having a large birefringence or optical anisotropy Δn. In this case, however, there arises a problem that the viewing angle is narrowed due to the birefringence of liquid crystal. For example, with the nematic phase liquid crystal, the polarized state of the crystal varies according to the incidence angle of light thereto. As a result, there arises a problem that the light is transmitted even in the black displaying state or the contrast degrades. Furthermore, if the product (Δn·d) of the optical anisotropy Δn and the cell thickness d is increased, there arises a problem that the viewing angle is narrowed.
Not to degrade the viewing angle characteristics even if the anisotropy Δn is increased, measures to collimate the incident light with microlenses is disclosed by the Japanese Non-Examined Patent Publication Nos. 4-369618 published in 1996 and 2000-171617 published in 2000. The measure or technique disclosed by the Publication No. 2000-171617 is schematically shown in FIG. 1.
In FIG. 1, the prior-art LCD device of the Publication No. 2000-171617 comprises a liquid crystal section 101, polarizer plates 104a and 104b located respectively on the input and output sides of the section 101, microlens arrays 142a and 142b located on the input and output sides of the section 101, a guide plate 105, and a light source 106. The array 142a has microlenses 142aa arranged at specified intervals on the upper surface, and windows or openings 142ab located at the corresponding positions to the lenses 142aa on the lower flat surface. The array 142b has microlenses 142ba arranged at specified intervals on the lower surface. The upper surface of the array 142b is flat.
The incident light LIN, which is introduced into the guide plate 105 through its side face, enters the microlens array 142a through the windows 142ab provided at its flat lower surface. The light LIN is collimated by the microlenses 142aa of the array 142a and then, converted to the linearly polarized light by the polarizer plate 104a and enters the liquid crystal section 101. The polarization direction of the light LIN is changed in the section 101. Thereafter, the light LIN thus direction-changed is selectively penetrated through the polarization plate 104b and collected by the microlenses 104ba of the array 104b, resulting in the output light LOUT emitted through the upper flat surface of the array 142b. 
In this way, with the prior-art LCD device of FIG. 1, the liquid crystal section 101 is interposed between the microlens arrays 142a and 142b and therefore, the collimated light LIN is introduced into the section 101 even if the light LIN emitted from the source 106 is a diffused one. As a result, the problem that the contrast degrades dependent on the observation angle is suppressed.
However, even so, the linearly polarized light LIN, which is generated by the polarizer plate 104a on the input side, enters the liquid crystal section 1 and then, the light LIN is turned on and off by the polarizer plate 104b on the output side. Thus, placement between the microlenses 142aa and 142ba, alignment between the polarizer plates 104a and 104b, and the orientation of the liquid crystal molecules require high accuracy. This causes a problem that the fabrication yield of the LCD panel degrades. Moreover, another problem that luminance decreases in part of the pixel occurs. This is due to the relationship between the driving direction of the liquid crystal molecules and the linearly polarized light.
Particularly, if the above-described technique to introduce collimated light into the liquid crystal section 101 is applied to the homeotropic orientation mode where high contrast is easily available, the transmitted light will not have a desired intensity, unless the orientation of rotation or inclination of the liquid crystal molecules is controlled accurately.